A well drilled into a hydrocarbon-bearing subsurface formation, during an initial post-completion stage, commonly produces crude oil and/or natural gas without artificial stimulation, because pre-existing formation pressure is effective to force the crude oil and/or natural gas out of the formation into the wellbore, and up the production tubing of the well. However, the formation pressure will gradually dissipate as more hydrocarbons are produced, and will eventually become too low to force further hydrocarbons up the well. At this stage, the well must be stimulated by artificial means to induce additional production, or else the well must be capped off and abandoned. This is a particular problem in gas wells drilled into “tight” formations—i.e., where natural gas is present in subsurface materials having inherently low porosities, such as sandstone, limestone, shale, and coal seams (e.g., coal bed methane wells).
Despite the fact that very large quantities of hydrocarbons may still be present in the formation, it has in the past been common practice to abandon wells that will no longer produce hydrocarbons under natural pressure, where the value of stimulated production would not justify the cost of stimulation. In other cases, where stimulation was at least initially a viable option, wells have been stimulated for a period of time and later abandoned when continued stimulation became uneconomical, even though considerable hydrocarbon reserves remained in the formation. With recent dramatic increases in market prices for crude oil and natural gas, well stimulation has become viable in many situations where it would previously have been economically unsustainable.
There are numerous known techniques and processes for stimulating production in low-production wells or in “dead” wells that have ceased flowing naturally. One widely-used method is hydraulic fracturing (or “fraccing”). In this method, a fracturing fluid (or “frac fluid”) is injected under pressure into the subsurface formation. Frac fluids are specially-engineered fluids containing substantial quantities of proppants, which are very small, very hard, and preferably spherical particles. The proppants may be naturally formed (e.g., graded sand particles) or manufactured (e.g., ceramic materials; sintered bauxite). The frac fluid may be in a liquid form (often with a hydrocarbon base, such as diesel fuel), but may also be in gel form to enhance the fluid's ability to hold proppants in a uniformly-dispersed suspension. Frac fluids commonly contain a variety of chemical additives to achieve desired characteristics.
The frac fluid is forced under pressure into cracks and fissures in the hydrocarbon-bearing formation, and the resulting hydraulic pressure induced within the formation materials widens existing cracks and fissures and also creates new ones. When the frac fluid pressure is relieved, the liquid or gel phase of the frac fluid flows out of the formation, but the proppants remain in the widened or newly-formed cracks and fissures, forming a filler material of comparatively high permeability that is strong enough to withstand geologic pressures so as to prop the cracks and fissures open. Once the frac fluid has drained away, liquid and/or gaseous hydrocarbons can migrate through the spaces between the proppant particles and into the wellbore, from which they may be recovered using known techniques.
Another known well stimulation method is acidizing (also known as “acid fracturing”). In this method, an acid or acid blend is pumped into a subsurface formation as a means for cleaning out extraneous or deleterious materials from the fissures in the formation, thus enhancing the formation's permeability. Hydrochloric acid is perhaps most commonly as the base acid, although other acids including acetic, formic, or hydrofluoric acid may be used depending on the circumstances.
Although fraccing and acidizing have proven beneficial capabilities, there remains a need for new and more effective methods for stimulating production in oil and gas wells. In particular, there is a need for stimulation methods that are more economical than known methods; that can enable recovery of higher percentages of non-naturally-flowing hydrocarbons from low-permeability formations than has been possible using known stimulation methods; that do not entail the injection of acids or other chemicals into subsurface formations; and that do not require the introduction of proppants into the formation.
U.S. patent application Ser. No. 11/746,470 (Kosakewich—US 2008/0035345 A1) teaches well stimulation methods and related apparatus whereby a subsurface formation is fractured by injecting an aqueous solution (e.g., fresh water) into the formation and then inducing freezing such that the aqueous solution expands, thereby generating expansive pressures that widen existing formation cracks and fissures in the formation and/or cause new ones to form. This process causes rock particles in existing cracks and fissures to be dislodged and reoriented therewithin, and also causes new or additional rock particles to become disposed within both existing and newly-formed cracks and fissures. Thawing is then induced in the frozen formation, such that the aqueous solution drains from the formation. The particles present in the cracks and fissures act as natural proppants to help keep the cracks and fissures open in substantially the same configuration as created during the freezing step.
In certain embodiments, the methods of U.S. patent application Ser. No. 11/746,470 include the steps of:                (a) providing a string of return tubing having an upper end and a lower end;        (b) providing a string of supply tubing having an upper end and a lower open end, with the lower end of the supply tubing having expander means;        (c) disposing the return tubing string within the wellbore so as to position the lower end of the return tubing at a selected depth, and so as to form a well annulus between the return tubing and the wellbore;        (d) disposing the supply tubing string within the return tubing string so as to position the expander means at a selected depth, and so as to form a tubing annulus between the supply tubing and the return tubing, with the return tubing string having associated plug means sealing off the tubing annulus at a selected location below the expander means;        (e) ensuring that an aqueous fluid is present in the well annulus to a selected level above the depth of the expander means;        (f) initiating a freezing cycle by introducing a flow of liquid refrigerant into the supply tubing, such that the refrigerant passes through the expander means and resultantly vaporizes and flows into the tubing annulus, and continuing the flow of refrigerant to freeze the aqueous fluid in a zone adjacent the expander means and to freeze an adjacent first region of the formation; and        (g) initiating a thaw cycle by discontinuing the flow of refrigerant and allowing said first region of the formation to thaw.        
The freeze-thaw steps of U.S. patent application Ser. No. 11/746,470 may be carried out on a cyclic basis. Each additional freeze-thaw cycle will cause additional formation fracturing, plus the creation of additional natural proppant particles. The appropriate or most effective number of freeze-thaw cycles in a given application will depend on a variety of factors including the physical properties of the formation materials.
In certain embodiments of the method of U.S. patent application Ser. No. 11/746,470, means are provided for subjecting the subsurface formation to LF wave energy during the freezing cycle of the method. This reduces the time required for each freezing cycle, for a given extent of penetration of the freezing front into the formation, thereby reducing the total time required for the well stimulation operation, thus enabling the well to be returned to production sooner.